Celestial Mechanics

Rapidly rotating planets and satellites have appreciable equatorial bulges as a consequence of Newton's First Law of Motion. If the rotation axis of such a body is not perpendicular to its orbit, other bodies in the system will exert stronger gravitational attractions on the near part of the bulge than its far part. The effect of this difference is to tend to turn the body's rotation axis perpendicular to the plane of its orbit. Because the body is rotating rapidly, however, this does not happen, and, like the rotation axis of a spinning top, the body's rotation axis describes a cone in space whose axis is the perpendicular to the body's orbit (in a two-body system). This phenomenon is called precession, and it is important for Earth, Mars, and the Jovian planets. For Earth, precession causes its celestial poles to describe small circles of 23.°5 arc radii around its ecliptic poles and the equinoxes to move westward on the ecliptic. They require 25,800 years to make one 360 circuit around the ecliptic poles and the ecliptic. For liars, the estimated period of precession is about 175,000 years.